1. Field of the Invention
The present invention relates to electromechanical transducers useful for converting mechanical movements or displacements into electrical signals. More particularly, the present invention concerns an improved piezoresistive transducer having an integrally formed frangible link for trimming the circuitry thereon.
2. Description of the Prior Art
Known electromechanical transducers, whether used for measuring acceleration, force or pressure, include a pressure/force sensitive element such as a diaphragm or cantilever and a strain sensing element attached to or integral with the pressure/force sensitive element.
Known transducers include unbonded wire transducers which consist of one or more wires stretched and connected between two or more points on a diaphragm and unsupported between these points. Displacement of the diaphragm stretches the wire reducing its cross-sectional area, and accordingly, increasing its resistance to the flow of electrical current in a proportional relationship to the diaphragm displacement. Also known are transducers wherein strain gages are bonded to the diaphragm using adhesives such as epoxy. These strain gages are sometimes made of thin copper alloy foil which is etched to define a gage geometry during manufacture. The foil strain gage is bonded over its entire length to the diaphragm and is not suspended therefrom. Here again, deflection of the diaphragm causes a strain in the strain gage element which changes its cross-sectional area and, accordingly, changes its resistance to the flow of electrical current.
Strain gage transducers are generally less expensive and more rugged than the larger unbonded wire transducers. Both types of transducers, however, have low gage factors (change in electrical resistance with respect to changes in strain in the gage element) and both may produce hysteresis errors affecting accuracy, when the gage is subject to periodic changes in strain.
Improvements are provided by piezoresistive transducers using a single-crystal semiconductor material, such as silicon doped with boron, for a strain gage element. The semiconductor strain gage is bonded to the diaphragm and offers the advantage of high gage factor (as much as one hundred times more sensitive than metal strain gages) and small size.
Improvements have also been provided by selectively changing the thickness of the diaphragm so that it will consist of relatively thick islands and a relatively thick rim portion, separated by thin portions. Because the deflection of the diaphragm is primarily along the thin portions, the strain gages can be advantageously mounted across the thin portions in an area of maximum strain to provide increased gage sensitivity.
U.S. Pat. No. 4,093,933 to Wilner teaches a transducer structure having a pressure diaphragm composed of a nonmetallic material that has been sculptured by etching to form thick islands and a thick rim interconnected by thin sheet material. The thick portions are separated by the thin flexures in which the deflection of the diaphragm is concentrated. Piezoresistive semiconductor sensors, similar to the type described in U.S. Pat. No. 3,351,880, are bonded on opposite sides of the thin grooves and electrical connectors are metallurgically bonded to the pads of the sensors so that the sensors may be connected in a wheatstone bridge type circuit. The sculptured pressure diaphragm is desirable and advantageous because it allows mounting the solid state strain gage elements across the points of maximum deflection which in essence mechanically amplifies the strain being sensed or measured as the diaphragm responds to forces. This structure is highly desirable because of its increased sensitivity when compared to structures wherein the strain gage element is mounted directly to the diaphragm surface along the entire length of the strain gage element.
Transducers formed from a single crystal of semiconductor material wherein the strain gages are an integral part of the semiconductor diaphragm are known in the art. These transducers offer the advantage of being small in size while having a high gage factor and are easier to manufacture. Also, the integral structure eliminates the need for an adhesive joint between the diaphragm and the strain gage. The adhesive joint is a disadvantage because it may not be as stable as the remainder of the crystal structure, and there can be relative movement, or creep, between the portions joined by the adhesive. However, these transducers, theoretically, lack the sensitivity of transducers with a sculptured diaphragm having piezoresistive semiconductors secured across slots in the diaphragm.
In U.S. Pat. No. 4,498,229, Wilner teaches an improved piezoresistive transducer and method for making same, wherein the gages are defined upon the substrate and subsequently etched from the material of the substrate. In this piezoresistive transducer the gage element is etched free of the substrate at its midportion and is integral with the substrate at its remote end portions. This piezoresistive transducer provides all the advantages of higher sensitivity provided by the sculptured pressure diaphragm and the freed gage structures described above and it also has the advantages of the one-piece integral strain gage/diaphragm structures described above.
Along other lines, it is well known that it is advantageous, in certain situations, to produce devices having electronic circuitry which can be altered after they are manufactured, to be made more suitable for the application involved. More specifically, it is common practice to trim integrated circuits by breaking or destroying a portion of the circuit to change the electrical performance characteristics of the entire circuit. The trimming of integrated circuits is sometimes performed using expensive and complex laser devices which focus laser energy on the portion of the circuit which will be severed or destroyed. Also, in metallic circuits, a portion of the circuit may be disconnected by providing an amount of electrical energy through the portion of the circuit that will cause it to self-destruct by melting. Further, circuits can be trimmed using devices which focus ultraviolet radiation on the portion of the circuit to be eliminated. All of the above-mentioned devices used for trimming circuits are expensive and in some cases difficult to use. It is desirable to have circuits which can be trimmed by relatively simple and inexpensive mechanical devices which, if the circuit is constructed accordingly, can mechanically destroy or disconnect that portion of the circuit to alter its characteristics.
Although the single-crystal piezoresistive transducer with etch-freed strain gages is known, there is still a need for piezoresistive transducer having etch-freed gages and integrated circuits which are capable of being trimmed by using simple and inexpensive mechanical means.